668 PROCEEDINGS OF THE CORDILLERAN SECTION 



rise to beuclies* or small valleys with one side a fault scarp, that scarp most 

 commonly showing a throw in like sense to the primary fault. In the Virginia 

 range, where there are probably two primary faults, a graben was found 

 bounded both east and west by fault scarps in part brolcen into steps. In some 

 places, instead of a scarp formed by movement along a single plane, are found 

 a series of step faults, which distribute the upper layer along the face of the 

 scarp, making it appear as very much thicker than normal, or giving rise on 

 distant view to doubt as to the existence of fault at that point. Such a front 

 is found on the Carson range not far from Reno and on parts of the Virginia 

 range. 



Lake Lahontajst 



The lower Truckee River valley and other valleys east of the Virginia range 

 were occupied by the Quaternary lake Lahontan and carry in their lower parts 

 the practically horizontal and unconsolidated lake beds. The sides of these 

 valleys are striated by the shorelines, with fresh cliffs, terraces, etcetera, and 

 are prominently coated by the Lahontan calcareous tufas already described by 

 Russell. t 



We may note that the lake beds are found about 12 miles up the Truckee 

 River canyon, showing that it had been cut even below its present level before 

 lake Lahontan was formed. In the upper part of the canyon the stream flows 

 on Tertiary volcanic bedrock. 



The Truckee meadows were not reached by the Lahontan water level, but 

 throughout that period, as at present, were occupied by the Truckee river and 

 its floodplains. Well developed terraces have been cut in the lake beds at least 

 up to 5,100 feet 5 miles west of Reno (700 feet above the river). Some of the 

 terraces have but a thin veneer of river deposit, but one about 80 to 100 feet 

 above the river is underlain by a thick deposit of coarse boulders. 



The Truckee River 



The Truckee river leaves the main Sierra front near Verdi, passes along the 

 axial belt of the depressed area (syncline) between the end of the Carson 

 range and the Peavine Mountain block, and flows out onto the main Truckee 

 meadows. Crossing these, it strikes right across the slope of the Virginia 

 range, which rises abruptly from the valley some 2,500 to 3,500 feet, and cuts 

 by a deep canyon almost pei-pendicularly across the range, emerging on the 

 depressed block to the east, where it turns north and follows the topographic 

 (orogenic) control to Pyramid lake. If we could fill up the Truckee canyon, 

 the river would flood the meadows and finally seek Pyramid lake to the north- 

 east by a shorter route. 



It seems very evident that the Truckee river is an antecedent stream, in 

 existence before the present ranges, and that the Virginia mountains have been 

 slowly raised aci'oss its course, the river being active enough to maintain its 

 course, and for that distance at least (25 miles) remain independent of the 

 orogenic control which is almost universal in this region. 



It is probable that during the gradual uprising of the mountains the Truckee 



• See figure illustrating subsidiary faulting in the Carson range, loc. cit., plate 16, and 

 discussion, p. .341. 



t U. S. Geological Survey, Moriograpli xi. 



